The preparation of aromatic carbonate polymers by passing phosgene into a pyridine solution of the aromatic compound is well known in the art. Pyridine has been used in the solvent polymerization of such polymers since it simultaneously acts to catalyze the reaction, acts as an acid acceptor for the hydrochloric acid reaction by-product and is also an excellent polymer solvent. Since pyridine is an expensive solvent, recovery thereof is essential in order for the process to be commercially practical. However, it is well established in the art that pyridine recovery from polycarbonate solutions is achieved only with difficulty. A water and caustic soda treatment can be used for the separation process but recovery of the pyridine hydrochloride therefrom is difficult and presents water and solid, e.g., sodium chloride, disposal problems. Moreover, in this treatment, the entire polymer solution must be treated and the desirable polymer properties may thus be affected. Other solvents alone or in combination with pyridine have been used to reduce costs; however, these methods have not simplified the separation and recovery problems.
In U.S. Pat. No. 3,144,432, for example, the use of chlorobenzene as a co-solvent with pyridine is taught. Chlorobenzene is a non-solvent for pyridine hydrochloride and about 95% of the formed pyridine hydrochloride is precipitated and can be removed by filtration. However, the remainder of the pyridine must be removed by distillation of the chlorobenzene-polymer mixture. The polymer is then recovered from the chlorobenzene solution by precipitation with a non-solvent or evaporation.
Many types of permeability separatory devices are known and a variety of separatory processes can be carried out therewith. See, for example, U.S. Pat. Nos. 3,228,876, 3,228,877, 3,422,008, 3,536,611, 3,556,991 and 3,637,467. General types of recovery operations, such as the recovery of solvents from organic solutions (column 16 of the '876 and '877 patents) and separation of various organic materials (column 6 of the '611 patent), and the specific recovery of metal salts from metal salt solutions (as in the '467 patent) are taught by these references. The '991 patent concerns the solvent extraction of aromatic hydrocarbons in conjunction with dialysis and fractionation of the extract to recover preferred extracts, such as benzene. British Pat. No. 1,437,969 teaches solvent extraction of a solute dissolved in a solvent by diffusing the solute across a membrane into a second solvent, said second solvent being immiscible with the first solvent.
However, none of the above references teach or suggest the problems or the solutions set forth herein.
It is therefore a primary object of the present invention to provide a simplified, highly efficient and non-aqueous method for the removal and recovery of pyridine hydrochloride from poly-carbonate solutions with a minimum amount of energy and cost.
It is a further object of the present invention to accomplish such pyridine hydrochloride removal by a continuous process involving a considerably improved method of separation, purification and recovery of the desired product as well as solvents and by-products.